A superconducting cable, which is one example of a superconductor, might lose its superconductive function and have its conductivity compromised due to a temperature rise caused by thermal load associated with the use and external heat intrusion. Thus, the superconducting cable, at the time of conducting electric power, needs to be constantly cooled to be maintained in an extremely low temperature state. One generally known method for cooling the superconducting cable employs circulative cooling using a sub-cooled coolant. This circulative cooling method using the sub-cooled coolant includes: cooling the coolant to be in a sub-cooled state with a refrigerator; transmitting the cooled coolant to the superconducting cable by a pump; and returning the coolant, having been used for cooling the superconducting cable, to the refrigerator.
However, the circulative cooling method using the sub-cooled coolant has the following risk. Specifically, when the refrigerator becomes faulty, the temperature of the sub-cooled coolant rises, and thus the temperature of the coolant for cooling the superconducting cable rises. As a result, the superconducting cable might lose the superconductive function and have its conductivity compromised. To overcome this risk, in one proposed method, a plurality of refrigerators are prepared. One of the refrigerators is operated in a normal state, and when this refrigerator becomes faulty, another one of the refrigerators is operated (see Japanese Patent Application Laid-open No. 2011-54500).